1. Field of the Invention
The present invention relates generally to a process for reducing harmful or unwanted emissions during the production of asphalt.
2. Description of the Prior Art
Vacuum tower operation for the production of asphalt binder requires the reintroduction of heavy vacuum gas oil (xe2x80x9cHVGOxe2x80x9d) to reduce entrainment and metals in the HVGO product. HVGO is reintroduced to the tower below the HVGO tray. A portion of the HVGO that is introduced below the tray is not vaporized and therefore survives to become part of the asphalt binder. The HVGO portion which survives increases the emissions when the asphalt binder is processed with aggregates in a hot mix plant. It is an objection of the invention to provide a process and apparatus that creates an asphalt binder which will produce decreased emissions when processed at a hot mix plant.
In order to avoid emissions and meet performance specifications, it is known in the art to add polymeric additives to the asphalt. The cost of these additives can be substantial. It is an object of the current invention to provide a process and apparatus for creating an asphalt binder that minimizes blue smoke without the addition of polymers. Blue smoke is defined or discussed in EPA guidance document AP 42 regarding emissions.
The American Association of State Highways and Transportation Organization (AASHTO) sets specifications for asphalts used in the paving industry. It is an object of the current invention to provide a process and apparatus for creating an asphalt binder that conforms to the AASHTO MP-1 performance specifications without the addition of polymers. Such asphalts are referred to as modified asphalts by virtue of its process.
Entrainment of heavy metals in gas oil vapors is problematic in the operation of such vacuum towers. It is an object of the current invention to reduce entrainment of heavy metals in the vacuum tower during the processing of asphalt.
The present invention is a method for reducing harmful or unwanted emissions during the production of asphalt. More specifically, the process reduces the production of xe2x80x9cblue smokexe2x80x9d when the asphalt binder is combined with aggregates in the hot mix plant. This xe2x80x9cblue smokexe2x80x9d is formed as a result of light ends or fractions which remain in the asphalt binder after processing in a vacuum unit. These light ends are released when the binder is heated in the hot mix plant. An alternate embodiment also reduces the amounts of entrained metals.
The xe2x80x9cpump aroundxe2x80x9d process of the current invention is used in the manufacture of asphalt binder and in conjunction with a vacuum tower or unit. The vacuum tower typically includes an HVGO tray or other middle distillate oil tray and an asphalt collector. By installing a collector tray between a middle distillate oil tray and an asphalt collector, a fractional cut of the material present between the trays is collected or removed from the vacuum tower and further processed before reintroduction into the system. The fractional cut or xe2x80x9cwax oilxe2x80x9d fraction is one of the cuts produced from the vacuum tower.
The current invention includes a process for separating a feed crude into fractional products that meet one or more of the above-identified objectives. The process includes feeding the feed crude into a pre-flash vessel to produce an overhead light ends product and a liquid flashed crude feed. The flashed crude feed is introduced into a heater to produce a partially vaporized crude feed. This partially vaporized crude feed is fed into a feed zone of a vacuum tower. From the vacuum tower is produced an overhead vapor stream, a middle distillate oil fraction, a wax oil fraction, and a bottoms asphalt product. The feed zone is located below a wax oil collection tray that collects essentially all liquids on the wax oil collection tray such that they are removed from the tower as the wax oil fraction. At least a portion of the wax oil fraction is heated and fed back into the feed zone. This process produces bottoms asphalt product that meets the performance specification desired.
An alternate embodiment includes introducing the portion of the heated wax oil fraction into a second flash vessel prior to introduction into the feed zone. Where the pre-flash vessel and the second flash vessel are used, one is typically under pressure while the other is under vacuum.
Another alternate embodiment includes feeding the wax oil fraction into the heater in combination with the liquid flashed crude feed. While the wax oil fraction is heated until it is essentially all vapor before reintroduction into the vacuum tower, the flashed crude feed is introduced in mixed phase.
Another embodiment includes feeding the wax oil fraction into the pre-flash vessel in combination with the feed crude such that the wax oil fraction is subjected to an additional vapor-liquid equilibrium separation prior to introduction into the feed zone of the vacuum tower.
It has been found that these embodiments of the current invention, while useful with many crude, provide particular advantage when the crude feed is Boscan Crude or Altimira Crude. The resulting product of this invention is the production of asphalt product that meets the specification for Performance Grade 76-22 conforming the AASHTO MP-1 specification. The asphalt product meets this specification without the addition of polymers or other additives. In this manner, the asphalt bottoms produced by all embodiments of this invention provide superior qualities even when the asphalt product is xe2x80x9cneatxe2x80x9d or without additives. Alternately, Performance Grade 70-28 or other desirable modified asphalts can be produced by virtue of the process that is one embodiment of the invention.
Heat can be added to the vacuum tower through traditional means such as through introducing steam into a bottom stripping section of the vacuum tower. Alternately, a reboiler or other traditional heat sources can be used to add heat to the stripping section of the vacuum tower.
A further alternate includes pumping a portion of the middle distillate oil fraction to a location in the vacuum tower above the middle distillate oil collection tray, referred to as a xe2x80x9cpump up.xe2x80x9d The middle distillate oil collection tray collects essentially all liquids on the middle distillate oil collection tray into the middle distillate oil fraction.
The invention also includes an apparatus designed for practicing this process. The apparatus for separating the feed crude into fractional products includes the pre-flash vessel to receive and separate the feed crude into overhead light ends product and liquid crude feed. The heater heats the liquid flashed crude feed to produce the partially vaporized crude feed. The vacuum tower has a feed zone to receive the partially vaporized crude feed. The feed zone is located below the wax oil collection tray, which collects essentially all liquids as a wax oil fraction. The asphalt product is removed from the bottom of the tower. The pump receives at least the portion of the wax oil fraction and introduces the portion of wax oil fraction into the feed zone.
In an alternate embodiment of the apparatus, the pump moves the portion of the wax oil fraction into the heater prior to the wax oil fraction portion being introduced in combination with the partially vaporized crude feed to the feed zone.
In another alternate embodiment of the apparatus, the pump moves the portion of the wax oil fraction into the pre-flash vessel in combination with the feed crude such that the wax oil fraction portion is flashed then heated prior to introduction into the feed zone.
Alternately, a second flash vessel receives the portion of the wax oil fraction prior to introduction of the portion of wax oil fraction into the feed zone. When the second flash vessel is used, this flash vessel is typically maintained at a different pressure than the pre-flash vessel.
The vacuum tower includes a bottom stripping section below the feed zone and has means for introducing steam to the bottom stripping section. Side to side stripping trays can be used below the wax oil collection tray and the feed zone to further reduce light components in the asphalt product. Other heating sources, such as reboilers and the like, can be used in place of or in addition to the means for introducing steam, also called steam stripping means.
In an alternate embodiment, the vacuum tower also includes the middle distillate oil collection tray. A pump-around pumps at least the portion of the middle distillate oil fraction to a location in the vacuum tower above the middle distillate oil collection tray, essentially all liquids on the middle distillate oil collection tray being collected into the middle distillate oil fraction.
This process and apparatus allow the heater to be run at a higher temperature which, in effect, reduces the amount of light ends left in the asphalt binder. In this way the entire process is continuous. By combining the wax oil with the crude feed prior to introduction to the vacuum unit, entrainment and metal contamination can be greatly reduced. Additionally, an asphalt binder is created which will produce fewer emissions when undergoing further processing.